Resistance, in electricity, is a property of any object or substance to resist the flow of an electrical current. The quantity of resistance in an electric circuit determines the amount of current flowing in the circuit for any given voltage applied to the circuit. The resistance of an object is determined by the nature of the substance of which it is composed, its resistivity, accounting for its dimensions and its temperature. Resistivity is expressed in terms of Ohms / cm3 at 20ºC. An electrical conductor is any material that offers little resistance to the flow of an electric current. The difference between a conductor and an insulator, which is a poor conductor of electricity, is one of degree rather than kind, because all substances conduct electricity to some extent. A good conductor of electricity, such as silver or copper, may have a conductivity billions of times as great, or more, as the conductivity of a good insulator, such as glass or mica.

Method

We were set the task of investigating the factors that come into play when determining the resistance of a piece of wire. We would be provided with the necessary apparatus needed to carry out the investigations. The basic setup would involve a circuit with a set of cells connected in series with an ammeter and the piece of wire being investigated, and a voltmeter connected in parallel with the wire. The ammeter is placed in series with the wire. An ammeter has a low resistance, so that it introduces as little extra resistance as possible into the circuit. The voltmeter is connected in parallel with the wire. Voltmeters have a high resistance, so the current they take is usually negligible.

We decided on the variables that could affect the resistance of a wire:
 Length of the wire

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...Submission
INSTRUCTOR: ENGR. JAY S. VILLAN, MEP-EE
Introduction
A multitester or multimeter is a device which can be used to gather data about electrical circuits. A basic multitester can measure resistance, voltage, and continuity; while more advanced versions may be able to provide additional data. This tool can be very useful to have around the house, and anyone who plans on doing electrical repairs should most definitely use a multitester for safety reasons. Multitesters can be used with the current off or on in most cases, although using the device with the current on can sometimes result in damage to the device.
Theory
Ammeters are employed for measuring current in a circuit and connected in series with the circuit. As ammeter is connected in series, the voltage drop across ammeter terminals should be as low as possible. This requires that the resistance of the ammeter should be as low as possible. The current coil of ammeter has low current carrying capacity whereas the current to be measured may be quite high. For this reason a low resistance is connected in parallel to the current coil.
Voltmeters are employed to measure the potential difference across any two points of the circuit these are connected in the parallel to the circuit. The resistance of voltmeter is kept very high by connecting a high resistance in series of the voltmeter with the...

...of this investigation is to expose the factors responsible for affecting the resistance of a wire in an electrical circuit. Many factors will have to be investigated prior to experimentation. A prior knowledge of electrical circuits and the factors of resistance will be required. The conclusive objective will be that research on the subject matter is proven by experimentation.
Resistance
The standard opinion of resistance when electricity is concerned is the ability of a substance to resist the flow of electricity through it. Good conductors are associated with low resistance and poor conductors are associated with high resistance. As resistance is responsible for the current that flows, a high resistance will be responsible for a low current and a low resistance will be responsible for a higher current.
This is definition of resistance given by Hutchinson&#8217;s Encyclopaedia:
&#8220;In physics, that property of a conductor that restricts the flow of electricity through it, associated with the conversion of electrical energy to heat; also the magnitude of this property. Resistance depends on many factors, such as the nature of the material, its temperature, dimensions, and thermal properties; degree of impurity; the nature and state of illumination of...

...d. Direct current
6. The current produced when pressure is applied to certain type of crystals is called
a. Electrostatic Induction b. Triboelatricity c. Photo electricity d. Pierelectricity
7. What will be the amount of current flowing when an emf of 10V is applied across an
open circuit
a. 10 Ampere b. 0 Ampere
c. The max current which source of emf can provide
d. none of the above
8. The accepted way of writing 4317 x 10 resistance is
a. 43 M7 b. 437 T c. 43 T 7 d. 4G 37
9. For a current carrying conductor, if length is doubled and area is halved the effect
on resistance will be
a. doubles b. one half
c. one fourth d. quadruple
10. In a color coded resistor, when red stripe is used in fourth band it does not
a. factor of 2 b. 3 % tolerance
c. multiplying factor of 100
d. none of the above
11. When two resistors are connected in parallel. Then which of the following will be true
a. same current flows through all parts of circuit
b. resistances are additive
c. branch currents are additive
d. voltage drops are additive
12. The two poles of an single magnet are always of equal strength and can exist in isolation
a. True b. False
13. The force of attraction between two opposite poles is
a. inversely proportional to the product of their pole strength
b. directly proportional to the square of the distance between them
c. directly...

...MasteringPhysics: Assignment Print View
http://session.masteringphysics.com/myct/assignmentPrint?assig...
[ Assignment View ]
Eðlisfræði 2, vor 2007
25. Current, Resistance, and Electromagnetic Force
Assignment is due at 2:00am on Wednesday, February 14, 2007
Credit for problems submitted late will decrease to 0% after the deadline has passed. The wrong answer penalty is 2% per part. Multiple choice questions are penalized as described in the online help. The unopened hint bonus is 2% per part. You are allowed 4 attempts per answer.
[
Current as Charge Flow
Electricity and Water Analogy
Learning Goal: To understand the analogy between water pressure, water flow, voltage, and current As suggested by the fact that we call both currents, the flow of charged particles through an electrical circuit is analogous in some ways to the flow of water through a pipe. When water flows from a small pipe to a large pipe, the flow (measured, for instance, in gallons per minute) is the same in both pipes, because the amount of water entering one pipe must equal the amount leaving the other. If not, water would accumulate in the pipes. For the same reason, the total electric current is constant for circuit elements in series. Water pressure is analogous to total electric potential (voltage), and a pump is analogous to a battery. Water flowing through pipes loses pressure, just as current flowing through a resistor falls to lower voltage. A...

...sad28078_ch02_023-046.qxd
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Resistance
No pain, no palm; no thorns, no throne; no gall, no glory; no cross,
no crown.
—William Penn
Historical Proﬁles
Georg Simon Ohm (1787–1854), a German physicist, in 1826
experimentally determined the most basic law relating voltage and current for a resistor. Ohm’s work was initially denied by critics.
Born of humble beginnings in Erlangen, Bavaria, Ohm threw himself intoelectrical research. Ohm’s major interest was current electricity, which had recently been advanced by Alessandro Volta’s invention
of the battery. Using the results of his experiments, Ohm was able to
deﬁne the fundamental relationship among voltage, current, and resistance. This resulted in his famous law—Ohm’s law—which will be covered in this chapter. He was awarded the Copley Medal in 1841 by the
Royal Society of London. He was also given the Professor of Physics
chair in 1849 by the University of Munich. To honor him, the unit of
resistance is named the ohm.
Ernst Werner von Siemens (1816–1892) was a German electrical
engineer and industrialist who played an important role in the development of the telegraph.
Siemens was born at Lenthe in Hanover, Germany, the oldest of
four brothers—all of whom were distinguished engineers and industrialists. After attending grammar school at Lübeck, Siemens joined the
Prussian...

...ABSTRACT
COOKSON, EDWARD JAMES. Development of the Metal Foam ElectricalResistance
Heater. (Under the Direction of Dr. Albert Shih.)
This thesis presents a novel concept using a radial heating element made from porous
Fe-Cr-Al metal foam in an air heater. Electricalresistance heating has been used extensively
to convert the electrical energy into thermal energy. An analytic heat transfer model is first
developed to estimate dimensions of the heating element. Four prototype Fe-Cr-Al metal
foam electrical heaters with different levels of porosity and density are built. A more
detailed computational fluid dynamics modeling of prototype heaters to include the
temperature loss to the surroundings is developed. Experiments were conducted to evaluate
effects of airflow rates and electrical current and measure the change of air inlet and outlet
temperatures. The temperature rise in the airflow is directly proportional to electric current,
and inversely proportional to the weight density of the foam. The temperature appears
directly proportional to airflow rate in low density foams, while it is inversely proportional in
foams of higher relative density. Experimental temperature measurements show reasonable
agreement with modeling predictions. Finally, possible improvements to the initial concept
are discussed.
Development of the Metal Foam Electrical...

...Resistance coursework
Aim: My aim is to find out which factors affect the resistance of wire and how they affect them.
Ohm's Law:
Ohm's law is also relevant to know of Ohm's Law, which states that the current through a metallic conductor (e.g. wire) at a constant temperature is proportional to the potential difference (voltage). Therefore V ¸ I is constant. This means that the resistance of a metallic conductor is constant providing that the temperature also remains constant. Furthermore, the resistance of a metal increases as its temperature increases. This is because at higher temperatures, the particles of the conductor are moving around more quickly, thus increasing the likelihood of collisions with the free electrons.
Resistance is the ratio of Voltage : Current and we calculate it by using the equation R=V/I.
Variables:
&#61594; Material of wire: In my experiment I will be using wire, because it has a high resistance. This could be either it has a closer ions or more ions than other metals. For example Copper has a low resistance due to the arrangement of its ions.
&#61594; Temperature: A rise in temperature causes ions inside the metal to vibrate more causing electrons to collide into them this builds resistance; therefore the electrons find it harder to get through the wire. The greater the temperature the greater the...

...Practice 1. Name a substance whose resistance almost remains unchanged by increase of temperature. 2. Name two special characteristics of heater coil. 3. A wire of resistance 4 ohms is bent to form a circle. What is the resistance between two diametrically opposite ends? 4. How does the resistance of a conductor change if its temperature is increased? 5. A current of 4A flows in a wire of resistance 60 ohms. Calculateelectrical energy consumed in 2 minutes. 6. V-I graph for two resistors is given. Which of the two has minimum resistance? 7. Alloys are used in electrical heating devices rather than pure metals. Give one reason. 8. An electric geyser has the ratings 2000W, 220V marked on it. What should be the minimum rating, in whole number of a fuse wire that may be required for safe use with this geyser? 9. The electrical resistivity of few materials is given below in ohm-meter. Which of these materials can be used for making element of a heating device? A. 6.84 x 10-8 B. 1.60 x 10-8 C. 1.00 x 10-4 D. 2.50 x 1012 E. 4.40 x 10-5 F. 2.30 x 1017
10. Where do we connect a fuse: with live wire or with neutral wire? 11. What is the resistance of an air gap? 12. Name two safety measures commonly used in electric circuits and appliances. 13. Two metallic wires A and B are connected in parallel. Wire A has length l and radius r, wire B has...